1. Field of the Invention
The present invention relates in general to a mobile telephone system using a wideband-code division multiple access (W-CDMA) system or the like, and more particularly to a method of controlling a transmission power of a mobile phone.
2. Description of the Related Art
In a mobile telephone system using the W-CDMA system, the control for an transmission power is required in order to transmit data between a base station (when it is assumed that one base station is constituted by one sector) and a mobile phone at a minimum electric power.
As for a method including a power control, the base station calculates a receiving power from a power control signal contained in transmission data from each of the mobile phones individually connected to the base station, and decides based on the calculation results whether a power of a signal transmitted from the mobile phone should be increased or decreased. The decision results are issued to the mobile phone through a power control signal (TPC) contained in the transmission data from the base station.
A signal frame #i of a separate down physical channel transmitted from a base station to a mobile phone of a code division multiple access (CDMA) system, as shown in FIG. 1, is composed of fifteen slots #0 to #14. Each slot #i includes data of a transport format combination indicator (TFCI), Data 1, transmit power control (TPC), Data 2 and Pilot. The mobile phone is controlled in accordance with TPC bits sent from the base station to which the mobile phone is being connected so that its transmission power is either increased or decreased.
That is to say, the mobile phone judges a transmission power when data is transmitted next time by using the TPC bits sent from the base station as a power control signal. In this power controlling method, when the base station is connected in one-to-one correspondence to the mobile phone, it is judged based on only the power control signal from the base station whether the transmission power from the mobile phone should be increased or decreased.
However, a plurality of base stations may be connected to a mobile phone in some cases. In such cases, the power control signals are respectively transmitted from a plurality of base stations to the mobile phone. Then, the mobile phone judges based on the power control signals from a plurality of base stations whether a power of a transmission signal outputted next time should be increased or decreased. According to the specification of a 3rd generation partnership project (3GPP) of the W-CDMA, the regulation are made such that in a case where a mobile phone is connected to a plurality of base stations, a transmission power is increased only when a request to increase the transmission power is made from all the base stations, and a transmission power is decreased when a request to decrease the transmission power is made from at least one base station.
For example, when a mobile phone is connected to two base stations as shown in FIG. 2, no misdetection of a power control signal is caused because a communication environment provided among a mobile phone 12, and base stations 10 and 11 are held in a good state. In other words, since there is no obstacle between the base stations 10 and 11, and the mobile phone 12, a plurality of information of effective paths 13 is transmitted to the mobile phone 12 without propagation loss.
On the other hand, as shown in FIG. 3, when there are obstacles 14 such as buildings between a base station 11 and a mobile phone 12, a radio wave including an effective path 13 is reflected by the obstacles 14. For this reason, information of the effective path 13 of a remarkably low receiving level is used between the base station 11 and the mobile phone 12.
At this time, when both the base stations 10 and 11 make a request to increase a transmission power from the mobile phone 12, the mobile phone 12 must increase its transmission power. However, while the mobile phone 12 can recognize that the transmission power should be increased in accordance with a power control signal from the base station 10, the mobile phone 12 can not properly detect a power control signal from the base station 11; because the receiving state of the power control signal from the base station 11 is poor. For this reason, there is a possibility that the mobile phone 12 may recognize that it must decrease its transmission power by mistake. As a result, al though both the base stations 10 and 11 make a request to increase the transmission power for the mobile phone 12, there is caused a situation in which the mobile phone 12 may carry out the control to decrease its transmission power. Consequently, there occurred deterioration of quality of the communication between the base stations 10 and 11, and the mobile phone 12, and disconnection of the communication.
In JP 2003-163630 A, such a problem is solved in a manner as will be described below. In regard to power control signals transmitted from all base stations and extracted by transmission power control information extracting means, when the power control signal from one base station instructs CDMA terminal equipment to decrease a power, and the power control signals from the remaining base stations instruct the CDMA terminal equipment to increase the power; the CDMA terminal equipment judges based on soft decision values of the power control signals from all the base stations whether its transmission power should be increased or decreased. Thus, there is avoided a problem that although all the base stations concerned in soft hand over (SHO) instruct a certain mobile phone to increase its transmission power, the transmission power is decreased due to a TPC bit error transmitted from one base station to deteriorate the communication quality of the mobile phone concerned.
However, in the technology described in JP 2003-163630 A, even the power control signal from the base station having a bad communication state is equally processed, which causes the CDMA terminal equipment to carry out misjudgment.
In addition, the soft decision processing is executed for deciding that the transmission power should be increased or decreased only when the power control signal from one base station among a plurality of base stations instructs the CDMA terminal equipment to decrease the power, and the power control signals from the remaining base stations instruct the CDMA terminal equipment to increase the power. As a result, when the power control signals from two or more base stations instruct the CDMA terminal equipment to decrease the transmission power, no soft decision processing is executed and thus, the transmission power is decreased.
Consequently, for example, in a state in which the CDMA terminal equipment is connected to three or more base stations, when the power control signal from the base station having the highest receiving level instructs the CDMA terminal equipment to increase the transmission power, but the CDMA terminal equipment recognizes that the power control signals from two or more base stations each having a lower receiving level instruct the CDMA terminal equipment to decrease the transmission power, the CDMA terminal equipment automatically decreases its transmission power. For this reason, there is also encountered a problem that the quality of the communication between the CDMA terminal equipment and the base station having the best communication state is deteriorated.